A goal of the manufacturers of thermocouple wire is to reliably produce thermocouple wire having thermoelectric properties which are as close as possible to a calibration standard. Currently, in the manufacture of thermocouple wire, it is necessary to modify the chemistry of the alloy to obtain the desired thermoelectric properties, thereby assuring an accurate measurement of temperature. This modification is more art than science, as the effect of small additions of reactive metals to the molten batch on the thermoelectric properties will vary with the melting techniques and the skill of the melter.
This problem is further aggravated by the fact that as an alloy is poured to form an ingot from which the wire is to be formed, the cooling of the alloy melt may go through two or more freeze temperatures starting at the periphery of the ingot and progressing to the center. For example, when solidifying a typical nickel-chromium alloy, such as used in thermocouple wires, an alloy consisting of 95% nickel and 5% chromium will solidify at the periphery while an alloy composition having 73% nickel and 27% chromium will solidify in the central region of the poured ingot. These differences in the chemical composition in various regions of the ingot often manifest themselves in thermoelectric properties of the final wire product. As a result, different segments of the drawn thermocouple wire may have different chemical compositions and different thermoelectric properties. Where the thermoelectric properties of the drawn wire are not within predetermined acceptable limits from a standard electro-motive force (emf) versus temperature curve, the entire ingot has to be scrapped. This is not only a waste of time and material, but also significantly reduces the production capability of the manufacture of thermocouple wires.